This invention relates to a voltage-to-current converter circuit, and, more particularly, to a voltage-to-current converter circuit suited for use as a constant current amplifier capable of driving a load by a constant signal current when the amplitude of an input signal is constant, regardless of value of the load.
Prior art voltage-to-current converter circuits will be described, by way of examples, with reference to FIGS. 1 to 3. In the circuit of FIG. 1, an operational amplifier 1 has its noninverting input connected to an input terminal 2 and its inverting input connected to circuit ground through a resistor 3 of a resistance value R.sub.11, and a load 4 of an impedance value Z.sub.L1 is connected between the output and inverting input of the operational amplifier 1.
When an input signal of voltage v.sub.i1 is applied between the input terminal 2 and circuit ground, output voltage v.sub.01 appearing between the output terminal 5 and circuit ground is given by ##STR1## where i.sub.01 represents current flowing through the load 4. Eliminating v.sub.01 from equations (1) and (2) and rearranging yields ##EQU2## Namely, the load current i.sub.01 is proportional to the input voltage v.sub.i1. However, the circuit of FIG. 1 is very difficult to use in designing circuits because the load is floated with respect to circuit ground.
In the circuit of FIG. 2, an operational amplifier 7 has its inverting input connected to an input terminal 8 through a resistor 9 (value R.sub.21), its noninverting input to circuit ground through a resistor 11 (value R.sub.23) and its output to its inverting input through a resistor 10 (value R.sub.22) and to its noninverting input through series connected resistors 12 (value R.sub.24) and 13 (value R.sub.25). A load 15 of impedance value Z.sub.L2 is connected between an output terminal 14 connected to a connection point of the resistors 12 and 13 and circuit ground. When an input signal of voltage v.sub.i2 is applied to the input terminal 8 load current i.sub.02 is represented by ##EQU3## where X.sub.1 =R.sub.21 (R.sub.23 +R.sub.25 +R.sub.24)-R.sub.23 (R.sub.21 +R.sub.22). When X.sub.1 =0 or ##EQU4## in equation (4), it will be seen that the load current i.sub.02 is proportional to the input voltage v.sub.i2 and does not depend on the load impedance Z.sub.L2. Although one terminal of the load 15 can be grounded, the circuit of FIG. 2 has a drawback that the input impedance is low because the operational amplifier 7 is used as an inverting amplifier. This drawback is eliminated by provision of an operational amplifier used as noninverting amplifier at the input side of the circuit of FIG. 2 as shown in FIG. 3.
In the circuit of FIG. 3, an operational amplifier 18 has its noninverting input connected to an input terminal 17 and its output connected to its inverting input through a resistor 19 (R.sub.31), and an operational amplifier 21 has its inverting input connected to the output of the operational amplifier 18 through a resistor 20 (R.sub.32), its noninverting input grounded and its output connected to its inverting input through a resistor 22 (R.sub.33) and to the inverting input of operational amplifier 18 through series-connected resistors 23 (R.sub.34) and 25 (R.sub.35). A load 26 (Z.sub.L3) is connected between an output terminal 24 connected to a connection point of the resistors 23 and 25 and circuit ground. When, in the circuit of FIG. 3, an input signal of voltage v.sub.i3 is applied to the input terminal 17, the load current i.sub.03 flowing through the load 26 is represented as follows: ##EQU5## When X.sub.2 =0, therefore, the load current i.sub.03 is proportional to the input signal voltage v.sub.i3 and does not depend on the load impedance Z.sub.L3. With the circuit of FIG. 3, although the input impedance is made high by provision of the noninverting amplifier 18, the output voltage signal is opposite in phase to the input voltage signal and the signal-to-noise ratio is deteriorated as compared with the circuit of FIG. 2 because of addition of the amplifier 18 having active devices. Generally, a circuit in which an output signal is in phase with an input signal is easy to use in designing circuits.